Known mooring systems for floating structures comprise a number of anchors, weights and associated tether lines which are attached to the installation or structure. The anchors can take different forms, but each is intended to hold its associated tether line on the bed of the body of water upon which the structure is floating. Additional weights can be added to the anchor lines in order to counter the forces on the structure (e.g. wind, waves or tidal currents) and maintain the position of the structure on the water. Additional mooring and restraint lines can also be incorporated depending on the conditions which the structure is expected to encounter.
One problem with such known mooring systems is that they present problems when the structure has to be retrieved from the water. With the number of tether and restraint lines attached to the structure, it is necessary for these all to be disconnected before the structure can be removed from the water. This normally requires human intervention, with an operator having to climb onto the structure in order to detach the various lines before the structure can be retrieved. In particularly heavy seas or bad weather, this cannot sometimes be done for safety reasons. The retrieval of the structure can therefore be delayed, for days at a time in some instances. Such delay can have serious time and cost implications, with maintenance or installation schedules being affected because of the delays.
A further disadvantage of the known systems is that they are normally intended to moor only one structure per system. If a number of structures are to be installed at the same location, separate anchor points, weights and tether lines are needed for each structure. As a result, the structures need to be a certain distance from one another to avoid fouling the various lines. Having to space the structures from one another in this manner is not an effective use of space. It is often desired to place a number of structures in as small a space on the water as possible, normally for economic, aesthetic or environmental reasons.
Another disadvantage is that known mooring systems do not make efficient use of the available water depth. The excursion required to cope with extreme waves and currents require large ranges of motion before the mooring lines become taut. If mooring lines become taut loads rise extremely fast—a process known as line snatching. This problem becomes serious if the structure is to be moored in shallow water and large waves and/or strong currents. Also, known systems do not make efficient use of the suspended weight of the mooring lines or components. Suspended weight costs money and may itself impart loads on the structure that may be large or undesirable.
Yet another disadvantage is that known mooring systems do not provide for ready, protected connection of the structure to one or more utility supplies such as power, fluid or water. Typically such connection must be made on or near the water surface, independently of mooring line connections.
In addition, a lot of known mooring systems are limited in that they are only intended to encounter loading from a single direction (e.g. a marine structure which is only intended to encounter waves from one direction). Such systems can lack stability if they encounter forces from an unexpected direction. Such forces can lead to rotational or yaw movements of the structure which can cause damage to both the structure and the components of the mooring system.
It is an aim of the present invention to obviate or mitigate one or more of the disadvantages of these known mooring systems.